docs: def exportMap(self,
                    bbox,
                    size="600,550",
                    dpi=200,
                    imageSR=None,
                    image_format="png",
                    layerDefFilter=None,
                    layers=None,
                    transparent=False,
                    timeFilter=None,
                    layerTimeOptions=None,
                    dynamicLayers=None,
                    mapScale=None
                    ):
          """
             The export operation is performed on a map service resource.
             The result of this operation is a map image resource. This
             resource provides information about the exported map image such
             as its URL, its width and height, extent and scale.
             Inputs:
              bbox - envelope geometry object
              size - size of image in pixels
              dpi - dots per inch
              imageSR - spatial reference of the output image
              image_format - Description: The format of the exported image.
                               The default format is .png.
                               Values: png | png8 | png24 | jpg | pdf | bmp | gif
                                       | svg | svgz | emf | ps | png32
              layerDefFilter - Description: Allows you to filter the
                               features of individual layers in the exported
                               map by specifying definition expressions for
                               those layers. Definition expression for a
                               layer that is published with the service will
                               be always honored.
              layers - Determines which layers appear on the exported map.
                       There are four ways to specify which layers are shown:
                          show: Only the layers specified in this list will
                                be exported.
                          hide: All layers except those specified in this
                                list will be exported.
                          include: In addition to the layers exported by
                                   default, the layers specified in this list
                                   will be exported.
                          exclude: The layers exported by default excluding
                                   those specified in this list will be
                                   exported.
              transparent - If true, the image will be exported with the
                            background color of the map set as its
                            transparent color. The default is false. Only
                            the .png and .gif formats support transparency.
                            Internet Explorer 6 does not display transparency
                            correctly for png24 image formats.
              timeFilter - The time instant or time extent of the exported
                           map image.
              layerTimeOptions - The time options per layer. Users can
                                 indicate whether or not the layer should use
                                 the time extent specified by the time
                                 parameter or not, whether to draw the layer
                                 features cumulatively or not and the time
                                 offsets for the layer.
                                 see: http://resources.arcgis.com/en/help/arcgis-rest-api/index.html#/Export_Map/02r3000000v7000000/
              dynamicLayers - Use dynamicLayers parameter to modify the layer
                              drawing order, change layer drawing info, and
                              change layer data source version for this request.
                              New layers (dataLayer) can also be added to the
                              dynamicLayers based on the map service registered
                              workspaces.
              mapScale - Use this parameter to export a map image at a specific
                         scale, with the map centered around the center of the
                         specified bounding box (bbox).
  
          """
          params = {
              "f" : "json"
          }
  
          if isinstance(bbox, Envelope):
              vals = bbox.asDictionary
              params['bbox'] = "%s,%s,%s,%s" % (vals['xmin'], vals['ymin'],
                                                vals['xmax'], vals['ymax'])
              params['bboxSR'] = vals['spatialReference']
              if dpi is not None:
                  params['dpi'] = dpi
              if size is not None:
                  params['size'] = size
              if imageSR is not None and \
                 isinstance(imageSR, SpatialReference):
                  params['imageSR'] = {'wkid': imageSR.wkid}
              if image_format is not None:
                  params['format'] = image_format
              if layerDefFilter is not None and \
                 isinstance(layerDefFilter,
                            filters.LayerDefinitionFilter):
                  params['layerDefs'] = layerDefFilter.filter
              if layers is not None:
                  params['layers'] = layers
              if transparent is not None:
                  params['transparent'] = transparent
              if timeFilter is not None and \
                 isinstance(timeFilter, filters.TimeFilter):
                  params['time'] = timeFilter.filter
              if layerTimeOptions is not None:
                  params['layerTimeOptions'] = layerTimeOptions
              if dynamicLayers is not None and \
                 isinstance(dynamicLayers, DynamicData):
                  params['dynamicLayers'] = dynamicLayers.asDictionary
              if mapScale is not None:
                  params['mapScale'] = mapScale
              exportURL = self._url + "/export"
              return self._get(url=exportURL,
                                  param_dict=params,
                                  securityHandler=self._securityHandler,
                                  proxy_url=self._proxy_url,
                                  proxy_port=self._proxy_port)
          else:
              return None